In a power transmission or distribution network, switching apparatuses are incorporated into the network to provide automatic protection in response to abnormal load conditions or to permit opening or closing (switching) of sections of the network. The switching apparatus may therefore be called upon to perform a number of different operations such as interruption of terminal faults or short line faults, interruption of small inductive currents, interruption of capacitive currents, out-of-phase switching or no-load switching, all of which operations are well known to a person skilled in the art.
In switching apparatuses the actual opening or closing operation is carried out by two contacts where normally one is stationary and the other is mobile. The mobile contact is operated by an operating device which comprises an actuator and a mechanism, where said mechanism operatively connects the actuator to the mobile contact.
Actuators of known operating devices for medium and high voltage switches and circuit breakers are of the spring operated, the hydraulic or the electro-magnetic type. In the following, operating devices will be described operating a circuit breaker but similar known operating devices may also operate switches.
A spring operated actuator or spring drive unit as it is also called, generally uses two springs for operating the circuit breaker; an opening spring for opening the circuit breaker and a closing spring for closing the circuit breaker and re-loading the opening spring. Instead of just one spring for each one of the opening spring and the closing spring, sometimes a set of springs may be used for each one of the opening spring and the closing spring. For example, such a set of springs may include a small spring arranged inside a larger spring or two springs arranged in parallel, side by side. In the following, it should be understood that when reference is made to the spring of the respective opening spring and the closing spring, such a spring could include a set of springs. Another mechanism converts the motion of the springs into a translation movement of the mobile contact. In its closed position in a network the mobile contact and the stationary contact of the circuit breaker are in contact with each other and the opening spring and the closing spring of the operating device are charged. Upon an opening command, the opening spring opens the circuit breaker, separating the contacts. Upon a closing command the closing spring closes the circuit breaker and, at the same time, charges the opening spring. The opening spring is now ready to perform a second opening operation if necessary. When the closing spring has closed the circuit breaker, the electrical motor in the operating device recharges the closing spring. This recharging operation takes several seconds.
Illustrative examples of spring operated actuators for a circuit breaker can be found e.g. in U.S. Pat. Nos. 4,678,877, 5,280,258, 5,571,255, 6,444,934 and 6,667,452.
At actuation of the switching apparatus, the moving contact part thereof is brought to a very high speed in order to break the current as fast as possible. At the end part of the movement it is important to decelerate the movement to avoid impact shocks. Therefore actuators of the kind in question normally are equipped with some kind of dampers to slow down the speed of the moving contact at the end of its movement. One damper is provided for the opening and one for the closing. Normally the dampers are linear with a piston operating in a hydraulic cylinder.
Such a damper is space-consuming and requires a plurality of components to be connected to the drive mechanism of the actuator.
In order to overcome such drawbacks and to provide a damper for the closing that requires small space and few components, EP 2317530 suggests using a rotary air damper for damping the closing. The device according to this disclosure thereby is more reliable and precise.
Although the operation of the device according to EP 2317530 has been shown to operate more reliable and precise than traditional devices of this kind, it has been found that the behavior of the damper at the end of the actuating stroke is critical for a proper performance. The air trapped within the working chamber after the displacement wall has passed the air outlet builds up a very high pressure when the sealing is good, which might cause back-bouncing of the displacement wall with operation failure as a consequence. This is explained more in detail in the specific part of the description.
In order to avoid such disastrous over pressure it is known to arrange a relief valve in the displacement wall or in the stationary end wall or both, which relief valve opens air passage through the displacement wall and/or the stationary end wall when a certain over-pressure is exceeded. Providing such a valve, however adds to the complexity of the device and increases assembly time and cost. A risk of failure is also introduced.
Another known solution is to control the damping behavior at the end of the stroke by having a pattern of holes that are closed successively. This controls the air flow and thereby the build-up pressure. The disadvantage of this method is that there is always flow through the open holes until they are closed. This leads to a slower pressure build-up and therefore results in a larger stroke angle to reach the turn-around point.